For the first time, time-lapse footage reveals the nightmarish horror of the heat-induced behavior known as bleaching.

Corals are dying in ocean reefs worldwide, the victims of abnormally hot waters warmed by El Niño and climate change.

Scientists have long known that higher ocean temperatures set off a biological reaction called bleaching.

Heat-stressed corals expel the colorful symbiotic algae that provide them with food, and in doing so, the corals turn bone white and face potential starvation.

Now, Australian scientists have captured these death throes on video—and it’s horrible to behold.
The researchers placed specimens of the coral Heliofungia actiniformis into aquatic tanks that simulate their natural environment, and then they turned up the heat.
The footage reveals that the corals eject the algae through a process called pulsed inflation.

They expand their bodies to as much as 340 percent of their normal size before violently contracting and spitting out the tiny organisms over a period of four to eight days. Seen in time-lapse video, the sea creatures turn paler with each convulsion.

In the short term, bleaching does have its benefits.
“The rapid expulsion of the coral's algal symbionts during thermal stress … could very well increase H. actiniformis's chance of survival during abnormally high sea temperatures," says Luke Nothdurft, one of the investigating scientists, in a press release.
That’s because, over time, the heat causes the symbiotic algae to become toxic, and expelling the algae keeps the coral alive.

But if the water doesn’t cool enough to allow new algae to quickly recolonize the coral, the bleached reef will die.